Fourier transform spectrophotometer

With conventional infrared (IR) spectrophotometers with a monochromator it was difficult to access the region from 10 to 400 cm^-1 (far IR), hence the first Fourier transform spectrophotometers (TF) were designed for that region. In any case, this method today has been extended to devices that allow scanning the entire IR region and, in particular, the average IR, which is the one of greatest interest. Currently, IR-TF spectrophotometers have displaced monochromator (dispersive) spectrophotometers.

Principle of operation

Scheme of a Michelson interferometer.

This instrument is based on the principle of the Michelson interferometer, which works as follows: the radiation first hits a splitter or separator which splits the light beam into two equal parts (semi-reflective mirror). These two light beams interfere with the splitter later on their return trip when they are reflected on two other mirrors. One arranged opposite the path of the original beam (moving mirror 1) and the other perpendicular (fixed mirror 2). The sample is placed in this trajectory and then the IR detector (see figure).

The intensity resulting from the superposition of the two beams is measured as a function of the offset (s) of the mobile mirror in its displacement with respect to the intermediate position. The resulting graph (Intensity vs. Phase) is called an interferogram.

The Fourier transformation is used as a mathematical method for developing the obtained curve (interferogram) in series. The transform is constituted by the sum of sines and cosines of the different optical frequencies that make up the radiation. Thanks to a computer program, this tedious mathematical calculation is simplified and exact and fast results of the elemental frequencies contained in the interferogram are obtained. The Fourier transform (or Fourier series expansion) of the interferogram is the ordinary spectrum obtained by conventional IR apparatus.

In effect, the interferogram contains the complete absorption of the sample described for each wavelength by the corresponding decrease in light intensity. The simplest interferogram corresponds to a monochromatic radiation (a single frequency), obtaining a cosine function curve of the corresponding frequency. In any interferogram each point contains data for all the frequencies contained in the full spectrum and not for a single frequency as in the ordinary spectrum.

Thus the information of a cosine shaped signal at the detector (simplest interferogram) would be shown after the transform as a single line of a particular wave number (monochromatic light of a single frequency). But any common interferogram is the result of the combination of multiple frequencies that with TF it is possible to discover.

Advantages of the method

The advantages of this IR-TF method are basically two:

1. improve spectrum resolution
2. obtain greater sensitivity

The improvement in sensitivity is a consequence of a higher flow energy of the light beam until it reaches the detector and of the improvement of the signal/noise ratio (S/N) by interferogram averaging. The advance in sensitivity is so remarkable that by the time cheap and accurate Michelson interferometers could be technologically made, virtually all commercial IR spectrophotometers today are IR-TF.